Возникла реальная опасность.

Because the subject is опасность, which is a feminine singular noun in Russian.

In the past tense, Russian verbs agree with the subject in gender and number:

• masculine: возник
• feminine: возникла
• neuter: возникло
• plural: возникли

So:

• опасность возникла = danger arose / appeared

The ending -ла shows feminine singular past tense.

Возникла comes from the verb возникнуть, which usually means:

• to arise
• to appear
• to come up
• to emerge

In this sentence, Возникла реальная опасность means something like:

• A real danger arose
• A real danger emerged
• There was a real danger

So the idea is that the danger came into existence or became real/actual.

Because опасность is the subject of the sentence.

In Возникла реальная опасность, the thing that arose/emerged is реальная опасность. That makes it the subject, so it stays in the nominative case.

You can see this in the adjective too:

• реальная = feminine singular nominative
• опасность = feminine singular nominative

They match because the adjective agrees with the noun.

Because it modifies опасность, which is a feminine noun.

Russian adjectives must agree with the noun they describe in:

• gender
• number
• case

So:

• masculine: реальный
• feminine: реальная
• neuter: реальное
• plural: реальные

Since опасность is feminine singular nominative, the correct form is реальная.

Both are possible, but they sound a little different in emphasis.

• Возникла реальная опасность puts the focus on the fact that a danger arose
• Реальная опасность возникла puts more focus on the real danger as the topic

Russian word order is more flexible than English because case endings and verb forms carry a lot of grammatical information. Beginning with the verb can make the sentence sound more narrative or report-like.

So this word order is very natural, especially in written or formal narration.

Возникла is perfective.

Its infinitive is возникнуть. The imperfective partner is возникать.

• возникнуть = to arise, to come into being once, as a completed event
• возникать = to arise, to keep arising, to arise repeatedly, or to be in the process of arising

Here, perfective is used because the sentence describes a completed event: at some point, the danger appeared/emerged.

Compare:

• Возникла опасность. = The danger arose.
• Возникала опасность. = Danger was arising / danger would arise / danger arose repeatedly (depending on context)

Russian has no articles, so there is no direct equivalent of a or the.

So реальная опасность can mean:

• a real danger
• the real danger

Which one is meant depends on context. In this sentence, English usually translates it as a real danger.

Yes, in many contexts that is a very natural translation.

Literally, the sentence is closer to:

• A real danger arose
• A real danger emerged

But in smoother English, especially depending on context, people may say:

• There was a real danger

That translation captures the overall meaning, even though it is less literal.

Because the main verb here is not to be. The verb is возникла.

The structure is:

• возникла = arose / emerged
• реальная опасность = real danger

So the sentence is already complete without any form of быть (to be).

Russian often omits to be in the present tense, but that is not the issue here. This sentence simply uses a different verb altogether.

The dictionary form is опасность.

It means danger and is a feminine noun, even though it ends in a soft sign -ь. Many Russian feminine nouns end this way.

A learner should remember that nouns ending in -ь can be either masculine or feminine, so gender must often be learned individually. In this case:

• опасность = feminine

That is why the verb is возникла and the adjective is реальная.

The stress is on the last syllable:

• возниклá

And in реальная опасность, the stresses are:

• реáльная
• опáсность

So the whole sentence is pronounced roughly as:

• возниклá реáльная опáсность

Stress is important in Russian because it is not always predictable and can affect pronunciation a lot.